Chapter I Introduction

 

 

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1. GENERAL INTRODUCTION

1.1 Definition of Corrosion and its Importance

Corrosion can be defined in many ways. Some definitions are very narrow and

deal with a specific form of corrosion, while others are quite broad and cover many

forms of deterioration. The word corrosion refers to the degradation of a metal by its

environment. The simplest definition of corrosion refers corrosion as the deterioration

of a material or its properties by chemical or electrochemical reaction when

interacting with its surrounding environment. The technical definition of corrosion

given by International Standard Organization (ISO) worked out in collaboration with

International Union of Pure and Applied Chemistry (IUPAC) defined corrosion as the

“Physicochemical interaction between a metal and its environment which results in

changes in the properties of the metal and which may often lead to impairment of the

function of the metal, the environment or the technical system of which these forms a

part” [1, 2]. However, the above definition did not include non metallic materials,

which are also corroded when exposed to their environments. Therefore, an

alternative definition of corrosion was suggested which refers corrosion as “an

irreversible interfacial reaction of a material (metal, ceramic, and polymer) with its

environment which results in consumption of the material or in dissolution into the

material of a component of the environment” [3]. Corrosion is a natural process of

deterioration and most metals corrode on contact with water, acids, bases, oil and

salts. The main reason for metals to corrode is their tendency to return to their

naturally occurring forms which is accompanied by a decrease in the free energy of

the system.

 

 

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The importance of corrosion was recognized in the sixties when it was realized

that damage was being caused to the economics of the industrialized nation, resources

are being wasted by anti-metallurgical process and useful life of manufactured goods

were being reduced [4]. The corrosion, an inherently difficult phenomenon to

understand, is a widely researched subject as it involved issues pertaining to the

human life and safety, huge environmental and economic impact, and conservation of

materials. The effects of corrosion in our daily lives are both direct, in that corrosion

affects the useful service lives of our possessions, and indirect, in that producers and

suppliers of goods and services incur corrosion costs, which they pass on to

consumers. The environmental impact and economic losses are the prime motive for

much of the current researches in the field of corrosion. The economic losses due to

corrosion are divided into direct losses and indirect losses [5]. Direct losses are those

losses associated with the direct replacement of corroded industrial equipment,

machinery or their components such as pipelines, condenser tubes, and replacement of

various parts of equipments and plants. Perhaps most dangerous of all is corrosion

that occurs in major industrial plants, such as electrical power plants or chemical

processing plants. Plant shutdowns can and do occur as a result of corrosion. Indirect

losses are more difficult to assess and contains plant shutdown, loss of efficiency, loss

of products as well as contamination of products and over design.

1.2 Cost of Corrosion

Corrosion has a major impact on the economy of a nation. The economic

losses due to corrosion to the worldwide industry are staggering. It is estimated that

25% of the total product of the metal and alloys go waste due to corrosion. In every

 

 

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country each year industries are paying huge price for corrosion and that cost is rising.

It is estimated that the annual cost of corrosion in the United States is in the range of $

9 billion to $ 90 billion. These figures were confirmed by various technical

organizations, including the National Association of Corrosion Engineers. The first

comprehensive landmark study on losses due to corrosion was carried out in USA in

late seventies. The results of study showed that the total loss due to corrosion in the

year 1975 was $ 70 billion which was approximately 5% of Gross National Product

(GNP) of that year [6]. Another study conducted in the US estimated the annual direct

cost of corrosion to be staggering at $ 276 billion which is approximately 3.1% of the

nation’s GDP [7]. A report on the cost of corrosion in India estimated the annual

losses due to corrosion to be Rs 25,000 crores per year which works out to be 4% of

GNP [8]. Approximately one third of the cost of corrosion is avoidable and could be

saved by using corrosion resistant materials and application of state-of-the-art

corrosion control technologies. A number of other studies on the economic losses due

to corrosion carried out at various times and in various industrialized nations suggest

that corrosion consumes 3-5% of GNP of that particular nations.

1.3 Corrosion Measurement Units

The corrosion rate may be expressed as an increase in corrosion depth per unit

of time (penetration rate, for example, mils/yr) or weight loss per unit area per unit

time, usually mdd (milligrams per square decimetre per day) or the corrosion current

(mA/cm2). Though preferred SI unit of corrosion rate expression is mm/yr or inch/yr,

the expression mils per year (mpy) (a mil being a thousand of inches) is the most

widely used and desirable corrosion rate expression in USA.

 

 

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t A

W

ρ

534(mpy)rateCorrosion (1.1)

where, W is weight loss in mg; ρ is the density of specimen in g/cm3; A is the area of

specimen in sq. in. and t is exposure time in hrs.

1.4 Electrochemical Theory of Corrosion

The electrochemical theory is the only theory which is universally accepted and is

applicable to most of the corrosion processes. Corrosion in an aqueous or an

atmospheric environment involves an electrochemical process which involves the

transfer of electrons between a metal surface and an aqueous electrolyte solution. An

electrochemical process consists of an anode, a cathode, an electrolyte for ionic

conduction and electron path for electron conduction. A corrosion reaction, in general,

is the sum of two complementary electrode processes; an anodic process involving the

oxidation of the metallic material, making electrons available in the metallic phase

and a cathodic process which consumes the electron made available by the anodic

process, through a reduction reaction.

Anodic process: The anodic reaction in every corrosion reaction is the oxidation of a

metal to its ion which passes into solution:

M M n+ + ne– (1.2)

where, n is the valence of metal, e– indicates the electron, M generic metallic material

and Mn+ its ion which passes in to the solution. In cases where the metallic material

tends to form hydroxides, the anodic reactions are of following type:

M + nH2O M (OH) n + nH+ + ne– (1.3)

 

 

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Cathodic processes: The cathodic processes involve reduction reaction and are

indicated by the consumption of electrons. In aqueous solution various reduction

reactions are possible depending upon the environment, which occurs at the cathode.

The possible reduction reactions are as follows:

2H+ + 2e – → H2 (gas) (1.4)

O2 + 4H+ + 4e– → 2H2O (1.5)

O2 + 2H2O + 4e– → 4OH– (1.6)

2H2O + 2e– → H2 (gas) + 2OH– (1.7)

M 3+ + e– → M 2+ (1.8)

M 2+ + 2e– → M (1.9)

During corrosion process, more than one oxidation and one reduction reaction may

occur and the anodic and cathodic reactions occurring during corrosion are mutually

dependent.

1.5 Corrosion Prevention and Control Methods

Corrosion control is the application of engineering principles and procedures

to minimize corrosion to an acceptable level by the most economical method. The

various methods of corrosion control are as follows:

1.5.1 Materials selection

The selection of the proper metal or alloy for a particular corrosive service is a

common method of preventing corrosion. It is of major importance in the chemical

process industries. A criterion for selection of ferrous and non-ferrous materials in

 

 

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equipment of construction is largely dependent on stability for the intended service,

availability, ease of fabrication and cost- economics. Carbon and low alloy steels are

the most widely used materials of construction as they satisfy all the above criteria

i.e., they are easy to fabricate, easily available and are inexpensive [9]. The physical

and chemical properties of the oxide film formed on an alloy and the service

environment determine the corrosion resistance of the alloy. Stainless Steels are

extensively used in petrochemical plants because of the highly corrosive nature of the

catalysts and solvents that are often used [10]. The corrosion resistance of stainless

steels is due to the protective nature of the surface oxide film that forms a barrier

between the environment and the alloy. High silicon cast irons (with 14% Si) are

extremely corrosion resistant because of a passive surface layer of silicon oxide that

forms during exposure to many chemical environments [11]. Tantalum is resistant to

most acids at all concentrations and temperatures and is generally used under

conditions where minimal corrosion is required, such as implants in the human body.

High-nickel cast iron (with 13 to 36% Ni and up to 6% Cr) has excellent corrosion,

wear and high temperature résistance because of the relatively high alloy content [12].

Copper and copper-nickel alloys are widely used in marine application. Titanium is

also susceptible to crevice corrosion in hot seawater and other hot aqueous chloride

environments.

The choice of a corrosion resistant material is quite complicated and is

accomplished in several stages. There are also metallurgical factors, such as

crystallography, grain size and shape, grain heterogeneity, second phases, impurity

inclusions, and residual stress that can influence corrosion. Thus, alloying,

 

 

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metallurgical treatments, and mechanical treatments can greatly affect the corrosion

resistance of the resulting alloy.

1.5.2 Proper design

Corrosion control makes the greatest economic sense at the design phase. The

proper choice of materials and pro-active detailing can provide handsome savings of

maintenance dollars, at very little cost expense. The mistakes in plant design are the

most frequently cited cause (58%) of corrosion failure in chemical process industries.

Proper design for corrosion control must include consideration of proper material

selection, finish selection, drainage, sealants, galvanic coupling of materials,

application of corrosion-inhibiting compounds, access for maintenance, the use of

effective corrosion control programs in service, and consideration of environmental

issues. Designs that introduce local stress concentrations directly or as a consequence

of fabrication should be carefully considered. 

1.5.3 Protective coatings

The application of protective coating is one of the popular options of corrosion

control [13]. Essentially, protective coatings are a means for separating the surfaces

that are susceptible to corrosion from the factors in the environment which cause

corrosion to occur. They give long terms protection under a broad range of corrosive

conditions, extending from atmospheric exposure to full immersion in strongly

corrosive solution. The application of a surface coating is common with a number of

different products, ranging from electrical wiring to printed labels. The coatings are

also helpful in preventing rust or enhancing the function of the product, as in the case

 

 

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of coated cookware. One of the most common examples of protective coatings is

electrical wiring. The wires that actually carry the flow of current are covered with

polymer coatings designed to contain power generated by the wires. At the same time,

the coating protects the wires from exposure to any outside element that could cause a

short or corrode the wires. The protective coatings in themselves provide little or no

structural strength, yet they protect the materials so that the strength and integrity of a

structure can be maintained. The protective coatings are further classified into

following types:

1.5.3.1 Organic coatings

These coatings afford protection by providing a physical barrier between the

metal and the environment. It is most widely used protective coating and is being used

for protecting aluminium, zinc and carbon steel against corrosion. According to the

Department of Commerce Census Bureau, the total amount of organic coating

material sold in the United States in 1997 was 5.56 billion L, at a value of $ 16.56

billion. The total sales can be broken down into architectural coatings, original

equipment manufacturers (OEM) coatings, special-purpose coatings, and

miscellaneous paint products. These coatings can also contain corrosion inhibitors.

Organic coatings include paints, resins, lacquers, and varnishes. Heavy organic

coatings, such as mastics and coal tars, are sometimes used to protect aluminum

surfaces that are embedded in soils and concrete.

 

 

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1.5.3.2 Inorganic coatings

These coatings are also used to provide a barrier between the environment and

the metal. Inorganic coatings also include enamels, glass linings, and conversion

coatings. The treatments change the immediate surface layer of metal into a film of

metallic oxide or compound which has better corrosion resistance than the natural

oxide film and provides an effective base or key for supplementary protection such as

paints.

1.5.3.3 Metallic coatings

Metallic coatings are another type of coating which provide a barrier between

the metal substrate and the environment. In addition, metallic coatings can sometimes

provide cathodic protection when the coating is compromised. Metallic coatings and

other inorganic coatings are produced using a variety of techniques, including hot

dipping, electroplating, cladding, thermal spraying, and chemical vapor deposition.

1.5.4 Changing the environment

Environment also provides a versatile means for reducing corrosion. For

aqueous corrosion, the environment can be made less aggressive by removing

constituents or modifying conditions that facilitate corrosion. This can be achieved by

decreasing temperature, decreasing velocity, preventing access of water and moisture,

removing dissolve oxygen, and increasing pH for steel. However, this method is

limited to closed system in which changes in the corrosion medium can be tolerated.

 

 

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1.5.5 Cathodic protection

Cathodic protection can be applied in practice to protect metals, such as steel,

copper, lead, and brass, against corrosion in soils and in almost all aqueous media.

When an external electric current is applied to a metallic structure to be protected, the

corrosion rate can be reduced to practically zero. Under cathodic protection, the metal

can remain indefinitely in a corrosive environment without deterioration. Cathodic

protection was first suggested by Sir Humphrey Davy in 1824 that copper could be

successfully protected against corrosion by coupling it to iron or zinc [14]. The most

rapid development of cathodic protection was made in the United States of America

and by 1945, the method was well established to meet the requirements of the rapidly

expanding oil and natural gas industry, which wanted to benefit from the advantages

of using thin-walled steel pipes for underground transmission. It is often most cost

effective course of action for the corrosion protection. The method is now well

established and is used on a wide variety of immersed and buried facilities and

infrastructure, as well as reinforced concrete structures, to provide corrosion control.

Cathodic protection can be used effectively to minimize corrosion fatigue,

intergranular corrosion, stress corrosion cracking, dezincification of brass, or pitting

of stainless steels in seawater, or steel in soil. It can however be fairly expensive in

the consumption of electric power or the extra metals involved in controlling the

potential within this region. Cathodic protection is also applied to canal gates,

condensers, submarines, water tanks, marine piling, offshore oil-drilling structures,

chemical equipments, bridge decks, parking garages, and other reinforced concrete

structures. Cathodic protection is fundamental to preserving a pipeline's integrity.

Cathodic protection is a method of corrosion control that is achieved by supplying an

 

 

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external direct current that neutralizes the natural corrosion current arising on the

pipeline at coating defects. Current required to protect a pipeline is dependent on the

environment and the number and size of the coating defects. For a particular

environment, greater the number and size of coating defects, the greater the amount of

current required for protection. Coating plays an integral part in the functioning of a

pipeline's cathodic protection system. The principle involved in cathodic protection is

to change the electrode potential of the metallic structure so that it lies in the

immunity region as shown in Figure 1.1. Within this region the metal is in the stable

form of the element and therefore corrosion reactions are impossible. In electro-

chemical terms, there are two types of cathodic protection applying to a metal

structure:

1.5.5.1 Impressed current method

Impressed current cathodic protection (ICCP) technique is widely used for the

protection of buried pipelines and the hulls of ships immersed in seawater. A d.c.

electrical circuit is used to apply an electric current to the metallic structure. The

negative terminal of the current source is connected to the metal requiring protection.

The positive terminal is connected to an auxiliary anode immersed in the same

medium to complete the circuit. The electric current charges the structure with excess

electrons and hence changes the electrode potential in the negative direction until the

immunity region is reached. The layout for a typical impressed current cathodic

protection system is shown in Figure 1.2. ICCP is most specialized technology and

can be very effective if correctly designed and operated. Typical materials used for

anodes are graphite, silicon, titanium, and niobium plated with platinum. Coatings are

 

 

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often used in conjunction with ICCP systems to minimize the effect of corrosion on

marine structures. One of the difficulties in designing a combined coating and ICCP

system is that coatings deteriorate with time. Precious metals are used for impressed-

current anodes because they are highly efficient electrodes and can handle much

higher currents. Precious metal anodes are platinized titanium or tantalum anodes; the

platinum is either clad to or electroplated on the substrate. Impressed current systems

are more complex than sacrificial anode systems and mostly used to protect pipelines.

1.5.5.2 Sacrificial anode method

The principle of this technique is to use a more reactive metal in contact with

steel structure to drive the potential in the negative direction until it reaches the

immunity region. Figure 1.3 illustrates the principle, in which sacrificial metals used

for cathodic protection consist of magnesium-base and aluminum-base alloys and, to a

lesser extent, zinc. No external power source is needed with this type of protection

system and much less maintenance is required. These metals are alloyed to improve

the long-term performance and dissolution characteristics. Sacrificial anodes serve

essentially as sources of portable electrical energy. For cathodic protection of offshore

platforms, aluminum anodes, made from aluminum-zinc alloys, are the preferred

material [15]. Most offshore petroleum-production platforms use sacrificial anodes

because of their simplicity and reliability, even though the capital costs would be

lower with impressed-current systems. Magnesium anodes have been used offshore in

recent years to polarize the structures to a protected potential faster than zinc or

aluminum alloy anodes. Magnesium tends to corrode quite readily in salt water, and

most designers avoid the use of magnesium for permanent long-term marine cathodic

 

 

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protection applications. Zinc anodes are also used to protect ballast tanks, heat

exchangers, and many mechanical components on ships, coastal power plants, and

similar structures. In seawater, passivity can be avoided by alloying additions, such as

tin, indium, antimony, or mercury. The three most common types of sacrificial anodes

are: activated aluminum, zinc and magnesium. Aluminum is the most widely-used

material for anodes, as it has a higher current capacity in comparison to the other

metals. Magnesium should be considered when the chloride content is less than

10,000 ppm.

1.5.6 Anodic protection

Anodic protection is an electrochemical method of controlling corrosion and is

based on the phenomenon of passivity. Passivity refers to the loss of chemical

reactivity experienced by certain metals and alloys under particular environmental

conditions. This method is most often used in highly corrosive environment to protect

metal immersed in solution with uncommonly acidic or basic qualities. Anodic

protection can be used to control the corrosion of metals in chemical environments

that exhibit very interesting behavior when subjected to anodic polarization. To

anodically protect a structure a device called potentiostat is required. When the

potential of the working electrode relative to the reference electrode is controlled and

shifted in the more positive direction, the current required to cause that shift varies. If

the current required for the shift has the general behavior with respect to potential,

shown in Figure 1.4, the metal is termed active passive and can be anodically

protected. If the metal was in passive state its corrosion rate could be reduced. Anodic

protection is applicable only to metals and alloys which are readily passivated when

 

 

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anodically polarized and for which i passive is very low. Anodic protection has been

used for storage vessels, process reactors, heat exchangers, and transportation vessels.

The primary advantages of anodic protection are its applicability in extremely

corrosive environments and its low current requirements. It is not applicable, for

example, to zinc, magnesium, cadmium, silver, copper, or copper-base alloys.

1.5.7 Use of corrosion inhibitors

The application of corrosion inhibitors is another prevalent method used for

corrosion control in closed systems. Corrosion inhibitor may be defined as a chemical

substances which, when added in small concentration to an environment effectively

reduces the corrosion rate of a metal exposed to that environment [16]. Corrosion

processes, being surface reactions, can be controlled by inhibitors which adsorb on the

reacting metal surface. The term adsorption refers to molecules attached directly to

the surface, normally only one molecular layer thick, and not penetrating into the bulk

of the metal itself. In general, an inhibitor forms a protective film in situ by reaction

with the corroding surface and as a result, the rate of the anodic and/cathodic

reactions are retarded. Corrosion inhibition is reversible and a minimum concentration

of the inhibiting compound is required to maintain the inhibiting surface film. The

effectiveness of corrosion inhibitors is dependent on the metal to be protected as well

as on the operating environment. They reduce the corrosion rate by:

Increasing or decreasing the anodic and/or cathodic reaction

Decreasing the diffusion rate for reactants to the surface of the metal

Decreasing the electrical resistance of the metal surface

 

 

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Inhibitors are mostly used in three types of environments:

Re-circulating cooling water systems (e g., of internal combustion engines,

rectifiers, and cooling towers) in the range of pH 5 to 9.

Primary and secondary production of crude oil and subsequent process and

Pickling acid solution for the removal of dust and mill scale during the

production and fabrication of metals parts or post service cleaning.

The inhibitors can be classified into the following types:

1.5.7.1 Passivating (anodic) inhibitors

This type of inhibitors (e.g., chromates and phosphates) acts by preventing the

anodic reaction. The inhibitors are incorporated into the oxide film on the metal,

thereby stabilizing it and preventing further dissolution. They cause the anodic curve

of polarization to shift such that less current flows. They have the ability to passivate

the metal surface. The passivating-type inhibitors are very efficient and reduce

corrosion rates to very low values. Some alkaline compounds such as NaOH, Na3PO4,

and Na2B4O are known to facilitate passivation of iron by favoring adsorption of

dissolved oxygen. There are two categories of passivating inhibitors namely,

oxidizing anions and non-oxidizing anions. Oxidizing anions have the ability to

passivate metal in the absence of oxygen. Typical oxidizing anions are chromate,

nitrite and nitrate. Non-oxidizing ones such as phosphate, tungstate and molybdate

require oxygen to perform passivation. However, one major drawback of such

inhibitors is that in order to maintain sufficient passivation of the metal and thus

 

 

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providing sufficient inhibition, the concentration of the inhibitor must be kept well

above a critical or minimum concentration. If the concentration is below the minimum

value, it is likely that the metal, which is to be protected in to the first place, will

suffer from localized corrosion such as pitting. Phosphate is widely used as an

additive in boiler water or cooling circuits and in pickling baths for metals. These

inhibitors slow down anodic reaction by forming passive film on the metal surface in

presence of oxygen.

1.5.7.2 Precipitation inhibitors

This class of inhibitor (e.g., zinc salts) block the cathodic reaction by

precipitating at the cathode due to elevated pH values locally. They precipitate on the

metal surface, forming a protective barrier. Hard water is rich in magnesium and

calcium. When these salts precipitate on the metal surface, for example at the cathode

where the pH is higher, they establish a protection layer on the metal.

1.5.7.3 Cathodic inhibitors

Those substances, which reduce the cathodic area by acting on the cathodic

sites and polarize the cathodic reactions, are called cathodic inhibitors [17]. Cathodic

inhibitors reduce the rate of cathodic reaction namely, oxygen reduction in near

neutral environments and hydrogen evolution in acid solutions, respectively.

1.5.7.4 Organic inhibitors

The inhibitors used for protection of steel in acidic solution are generally

organic inhibitor. These types of inhibitors are film-forming in nature. They form a

 

 

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hydrophobic layer on the surface of the metal to prevent dissolution of metal. Most

organic inhibitors are substance, which possess at least one functional group

considered as the reaction center for the adsorption process. The adsorption of the

inhibitor is related to the presence of heteroatom such as nitrogen, oxygen,

phosphorus and sulfur as well as triple bond or an aromatic ring in their molecular

structure through which they can adsorb on the metal surface [18-21].The efficiency

of this type of inhibitors depends upon the chemical composition and molecular

structure of the compounds as well as their affinity with the metal. They are further

classified into organic anodic and cathodic or both depending on the reaction

mechanism and the potential of the metal at the interface [22].

1.5.7.5 Inorganic inhibitors

The common inorganic inhibitors used are crystalline salts, for instance,

sodium chromate and molybdate. The addition of heavy metal ions like Pb2+, Mn2+,

Cd2+ is found to inhibit corrosion on iron in acidic medium. This may be due to the

deposition of these metal ions over the iron surface [23].

1.5.7.6 Mixed inhibitors

Corrosion inhibitors are rarely used as a single compound. The formulation

can be composed of two or more inhibitors which will carry different characters. The

use of mixed inhibitors is due to following three main factors:

A single inhibitor can only inhibit a few numbers of metals. When the

environments involve multi-metal system, the inhibitive action may

sometimes cause jeopardizing effects to other metals.

 

 

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Advantages from anodic and cathodic inhibitors can be combined and

optimized for best performance.

Addition of halide ions improves the action of organic inhibitor in acid

solutions

1.5.7.7 Neutral inhibitors

These inhibitors include cathodic inhibitors, anodic inhibitors and mixed or

general inhibitors [24]. The compounds used in neutral media are borates, molybdates

and salts of organic acids like benzoates and salicylates.

1.5.7.8 Vapor-phase corrosion inhibitors

Vapor-phase corrosion inhibitors or volatile corrosion inhibitors (VCIs) are

similar to organic adsorption-type inhibitors. These inhibitors possess moderately

high vapor pressure and consequently can be used to inhibit atmospheric corrosion of

metals without applying VCIs directly on the metal surface. Volatile corrosion

inhibitors are secondary-electrolyte layer inhibitors that possess appreciable saturated

vapor pressure under atmospheric conditions, thus allowing vapor-phase transport of

the inhibitive substance” [25]. They are usually effective if used in enclosed spaces

such as closed packages or the interior of machinery during shipment [26]. Vapor

phase inhibitor function by forming a bond on the metal surface or by forming a

barrier layer to aggressive ions.

 

 

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1.6 Corrosion Protection of Steel in Acid Solutions Using Green Corrosion

Inhibitors

1.6.1 Introduction

Though all the known materials are susceptible to corrosion the subject of

steel corrosion has received considerable attention during the last few decades [27].

Mineral acids are widely used in various industries for pickling of steel at elevated

temperatures up to 60 oC. This technique besides being used to remove corrosion

scales from steel surface without causing acid attack of the bulk metal is also

effectively applied in cleaning of industrial equipment and acidization of oil well [28].

There are several method of preventing corrosion and the rates at which it can

propagate with a view of improving the lifetime of metallic and alloy materials. The

use of inhibitors is one of the most effective, practical and economic methods to

protect metallic surfaces against corrosion in aggressive acidic media [29-31]. Most of

the efficient pickling inhibitors are organic compounds containing hetero atoms such

as sulfur, nitrogen, oxygen, phosphorus, and multiple bonds or aromatic rings in their

structures [32, 33].The number of lone pairs of electrons and loosely bound π-

electrons in these functional groups are the key structural features that determine the

inhibitive action of these compounds [34, 35]. These compounds prevent corrosion by

blocking the active corrosion sites either by getting adsorbed, or by forming a

protective layer or an insoluble complex on the metal surface. The effectiveness of an

organic inhibitor depends mainly to its chemical structure and physiochemical

properties of the compounds like functional groups, aromaticity and/or presence of

conjugated bond, nature and number of bonding atoms [36]. In addition to this, other

 

 

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factors such as molecular size and the nature of the substituent groups and heteroatom

present in the organic compounds also govern the effectiveness of the compounds as

corrosion inhibitor [37-42]. Other considerations in selection of an inhibitor are the

cost of the inhibitor, its availability and environmentally acceptability. However, most

of the organic compounds used as corrosion inhibitors are toxic and hazardous to both

human beings and the environment [43] and needs to be replaced by nontoxic,

environment friendly compounds. As a result, the current research trends is towards

the development of nontoxic, economical and more environmentally safe green

chemicals as corrosion inhibitors [44-48]. Naturally occurring substances of both

plants and animal origin otherwise tagged ‘green inhibitors’ are known to meet these

requirements. In the past two decade, the research in the field of ‘‘green’’ or “eco-

friendly” corrosion inhibitors has been directed towards the goal of using cheap,

effective compounds at low or ‘‘zero’’ negative environmental impact [49-53].

1.6.2 Plant extracts as green corrosion inhibitors

In recent years, there is a considerable amount of effort devoted to develop

biodegradable and efficient green corrosion inhibitors extracted from natural plants.

Plant extracts, in addition to being environmentally friendly and ecologically

acceptable, are inexpensive, readily available and renewable. The extracts of Azaricta

indica, Fenugreek leaves, Zenthoxylum alatum, Opuntia, Nypa fruticans, Ocimum

viridis, Phyllanthus amarus, Chamomile, Halfabar, and Murraya koenigii were

studied as corrosion inhibitors in sulphuric and/or hydrochloric acid medium [54-60].

Chauhan and Gunasekaran [61, 62] studied the corrosion inhibition of steel by eco-

friendly Zenthoxylum alatum plant extract in HCl as well as in phosphoric acid

 

 

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medium. Other plant extract like opuntia [63], lawsonia [64] and khillah [65] have

been evaluated for corrosion inhibition of metals like Cu, Al, Zn and steel. Other than

the plant extracts, pure organic compounds extracted from natural products such as

ascorbic acid [66], succinic acid [67], caffeine [68], Pennyroyal oil [69], and caffeic

acid [70] have also been used for inhibition of corrosion. Eddy and Ebenso [71]

studied the adsorption and inhibitive properties of ethanol extract of Musa sapientum

peels as green corrosion inhibitor for mild steel in H2SO4 using hydrogen evolution

and thermometric methods. The different concentration of ethanol extract of M.

sapientum peels was found to inhibit mild steel corrosion. In a very recent paper the

corrosion inhibition by extract of Stevia rebaudiana leaves on mild steel in sulphuric

acid solution was reported [72]. The techniques used are electrochemical impedance

spectroscopy (EIS) and potentiodynamic polarization techniques. The inhibition,

which was assumed to occur via adsorption of the inhibitor molecules on the metal

surface, was found to increase with increasing concentration of the leaves extract. The

adsorption of the extract on the mild steel surface obeys the Langmuir adsorption

isotherm. Ouariachi et al [73] investigated the inhibition effect of Rosamarinus

officinalis oil as green corrosion inhibitor on C38 steel in 0.5 M H2SO4. Quraishi et al

[74] reported the corrosion inhibition properties of black pepper extract in mild steel

in HCl solution by weight loss, potentiodynamic polarization, and EIS. It was shown

that black pepper extract gave maximum inhibition efficiency of 98% at 120 ppm at

35 oC. Electrochemical measurements revealed it to be a mixed- type inhibitor. Noor

[75] investigated the temperature effects on the corrosion inhibition behavior of mild

steel in 2 M HCl and H2SO4 in the absence and presence of aqueous extract of

fenugreek leaves (AEFL) by gravimetric method. It was shown that the AEFL

 

 

22

inhibited mild steel corrosion in HCl more than in H2SO4 at all inhibitor

concentrations and solution temperatures. The inhibition action of AEFL was

performed via adsorption of the extract species on mild steel surface. The adsorption

was spontaneous and followed Langmuir adsorption isotherm in HCl, while followed

Temkin adsorption isotherm in H2SO4 at all studied temperatures.

The inhibition performance of plant extract is normally ascribed to the

presence of complex organic species including tannins, alkaloids and nitrogen bases,

carbohydrates and proteins as well as hydrolysis products in their composition. These

organic compounds usually contain polar functions with nitrogen, sulfur, or oxygen

atoms as well as triple or conjugated double bonds or aromatic rings in their

molecular structures, which are the major adsorption centers.

1.6.3 Polymers as green corrosion inhibitors

The use of polymers as green corrosion inhibitors have drawn considerable

attention recently due to their inherent stability and cost effectiveness. Owing to the

multiple adsorption sites, polymeric compounds adsorb more strongly on the metal

surface compared with their monomer analogues [76] Therefore, it is expected that

the polymers will be better corrosion inhibitors. The polymer inhibitors which have

been examined include: polyethylene glycol, polyvinyl alcohol, polyvinyl pyridine,

polyvinyl pyrrolidone, polyethylenimine, polyacrylic acid, poly acrylamide and starch

[77-82]. Umoren et al [83] studied the corrosion inhibition behavior of mild steel in

H2SO4 in the presence of naturally occurring polymer gum arabic (GA) and synthetic

polymer polyethylene glycol (PEG). It was found that PEG was more effective than

GA.

 

 

23

Studies on the use of some macromolecules or natural polymers which are

green inhibitors have been carried out. For instance, Eddy et al [84] carried out gas

chromatography-mass spectroscopy (GC-MS) study to elucidate the chemical

structure of Anogessus leocarpus gum (AL) and also investigated its corrosion

inhibition potential for mild steel in solutions of HCl. The analysis of AL indicated

the presence of sucrose (10.03%), phthalic acid (2.53%), n-hexadecanoic acid

(11.73%), oleic acid (30.49%), pentacenequinone (4.41%) and 2,3-

diphenylnaphthoquinone (21.43 %). The adsorption of the inhibitor on mild steel

surface was found to be exothermic, spontaneous, Langmuir type and supports the

mechanism of charged transfer from the inhibitor’s molecule to the charged metal

surface. Guar gum has been shown to be an effective corrosion inhibitor for some

metals in aggressive acid environment [85]. Inhibition efficiency was found to

increase with increase in the concentration of the tested material. Umoren et al [48]

reported the corrosion behavior of exudate gum from Raphia hookeri (RH) gum as a

potential eco-friendly inhibitor for mild steel in H2SO4 using weight loss and

hydrogen evolution techniques at 30-60 oC. The inhibition efficiency was observed to

increase with increase in RH concentration but decreased with rise in temperature,

which is suggestive of physical adsorption mechanism. The adsorption of the exudate

gum onto the steel surface was found to follow the Langmuir adsorption isotherm.

Corrosion inhibition of mild steel in hydrochloric acid by betanin as a green

inhibitor in 1 M HCl solution was studied by weight loss method, potentiodynamic

polarization, and electrochemical impedance spectroscopy techniques under the

influence of various experimental conditions [86]. The polarization curves showed

that betanin behaves mainly as a mixed-type inhibitor. Maximum inhibition efficiency

 

 

24

(98%) is obtained at betanin concentrations of 0.01 M. The results obtained from

weight loss, polarization, and impedance measurements are in good agreement. Obot

et al [87] studied the effect of Sulphathiazole (STZ) as green corrosion inhibitor at

mild steel in 0.5 M HCl at different temperature using gravimetric and quantum

chemical techniques. It was shown that STZ acts as inhibitor for steel corrosion even

at very low concentration. The adsorption process was found to obey the Temkin

adsorption isotherm. The calculated activation and Gibbs free energy values confirm

the chemical nature of the adsorption process. It is most probable that the inhibition

property of STZ was due to charge sharing between the inhibitor molecules and the

metal surface.

1.6.4 Amino acids as green corrosion inhibitors

It has been shown by a number of investigators that some amino acids can act

as corrosion inhibitors, which has generated an increasing interest in these compounds

[88-102]. Amino acids are attractive as corrosion inhibitors because they are relatively

easy to produce with high purities greater than 99% at low cost and are nontoxic,

biodegradable and completely soluble in aqueous media. Among the amino acids

sulfur containing amino acids has been found to be more efficient corrosion inhibitor

[103-109].

El-Nabey et al [103] reported the electrochemical measurements and corrosion

behavior of mild steel specimens immersed in 1 N H2SO4 solution in the presence and

absence of amino acids at room temperature. Inhibition increased with increasing

additive concentration and showed that adsorption of the amino acids on the steel

surface occurs in two steps: firstly a monolayer of the adsorbate is formed on the

 

 

25

metal surface, and this is followed by the deposition of a second adsorbate layer. The

corrosion inhibition of iron in 1 M HCl using twenty two different common amino

acids and four related compounds was investigated using potentiodynamic

polarization curves [110]. The best results were obtained with 3, 4-diiodotyrosine,

with an inhibition efficiency of 87%. The best common amino acid was tryptophan

with an inhibition efficiency of 80%. Hydroxyproline, cystine, and cysteine acted as

corrosion accelerators. In general, amino acids with longer hydrocarbon chains

showed greater inhibition. Additional groups or groups which increased electron

density on alpha amino group also increased the inhibition efficiency. Kalota and

Silverman [111] showed the corrosion inhibition of steel by aspartic acid. At pH less

than the ionization constant of aspartic acid it accelerated corrosion. Above that pH, it

acted as an inhibitor for the steel. Abdel et al [112] studied the corrosion inhibition

behavior of eleven amino acids namely, glycine, alanine, serine, phenylalanine,

tyrosine, tryptophan, histidine, aspartic acid, cysteine, cystine, and methionine on

mild steel corrosion in 3 M H2SO4 by electrochemical techniques. Among the studied

amino acids sulphur containing amino acids were found better corrosion inhibitors.

The inhibition efficiencies of cysteine, alanine, valine, phenyl alanine, serine and

threonine on the corrosion of steel in 2 M H2SO4 were studied by mass loss and

gravimetric technique at 25 oC [113]. It was showed that the inhibition efficiency

depends on the type of amino acids and its concentration. The inhibition efficiency

varies from 42% to 91%. Nineteen different naturally occurring amino acids including

methionine were studied as corrosion inhibitor for mild steel in H2SO4 using Tafel

polarization curves [104]. The best corrosion inhibition was obtained with the S-

containing amino acids. The inhibiting performance of five amino acids, on steel in 1

 

 

26

M H2SO4 solution, using potentiodynamic method was investigated [114]. The

maximum efficiency was obtained with glutamic acid.

The corrosion inhibition action of four amino acids containing sulfur namely,

cysteine, N-acetylcysteine, methionine and cystine on the corrosion of mild steel in

phosphoric acid solution polluted with Cl-, F- and Fe3+ ions were studied using

potentiodynamic and electrochemical impedance techniques [115]. Cysteine and N-

acetylcysteine showed higher inhibition efficiency than methionine and cystine. Both

F- and Fe3+ ions stimulated mild steel corrosion while Cl- ions inhibited the corrosion.

The inhibition effect of three amino acids namely, alanine, glycine and leucine against

steel corrosion in 0.1 M HCl solutions was investigated by potentiodynamic

polarization method [91]. Corrosion parameters such as corrosion rate, corrosion

potentials (Ecorr) and corrosion resistance (Rp) were determined by extrapolation of the

cathodic and anodic Tafel region. Adsorption isotherm was investigated by weight-

loss measurement. The effect of inhibitor concentration and acid concentration against

inhibitor action was investigated. The inhibition efficiency was found to depend on

the type of amino acid and its concentration. The inhibition effect of the amino acids

was found to range from 28-91%. The adsorption of amino acid followed Langmuir

adsorption isotherm. Moretti et al [116] investigated the inhibition effect of

tryptamine as green corrosion inhibitor for iron in 0.5 M deaerated H2SO4

(temperature range 25-55 oC) by means of potentiodynamic curves and

electrochemical impedance spectroscopy (EIS). Trypatamine was found to be an

effective ARMACO iron inhibitor, even at 55 oC and 72 h, but only at 10 mM. At this

concentration the inhibition percentage, calculated by potentiodynamic curves and

EIS, ranged from 90% to 99% and did not diminish with increase in time and

 

 

27

temperature. Tryptamine adsorption follow Bockris-Swinkels’ isotherm. The

thermodynamic data indicated that in the more concentrated solution, tryptamine also

chemisorbed on the iron surface. Abiola [106] showed the adsorption of methionine at

mild steel surface from acidic solution studied by using gravimetric technique. The

adsorbability of methionine was found to depend on the nature of the acid and

concentration of methionine. The surface coverage with the adsorbed methionine was

used to calculate the free energy of adsorption, ∆Goads using Bockris-Swinkel

isotherm. The dependence of ∆Goads on surface coverage q is due to heterogeneous

nature of the mild steel surface.

Alagbe et al [117] investigated the effect of different amino acid namely,

leucine, alanine, methionine and glutamic acid on the corrosion characteristics of

NST-44 mild steel in lime fluid (citrus aurantifolia) using weight loss immersion

method. The corroded surfaces of the specimens were characterized using optical

microscopic technique. Alanine showed the highest inhibitive potential on NST-44

mild steel in the lime fluid at lower concentration while glutamic acid showed the

least potential. Leucine and methionine, however, showed considerable potentials

with inhibition efficiencies of about 41 and 44.70%, respectively. Olivares et al [118]

studied the corrosion inhibition of steel in 1.0 M HCl by decylamides of α-amino

acids derivatives using gravimetric and electrochemical techniques. Protection

efficiencies of 90% were obtained with 100 ppm of tyrosine and glycine derivatives,

while alanine and valine derivatives reached only 80%. The order of increasing

inhibition efficiency was correlated with the modification of the molecular structure

of inhibitors. Potentiodynamic polarization curves indicated that both the decylamide

of tyrosine and glycine acted primarily as anodic type inhibitors, whereas the

 

 

28

decylamide of alanine and valine were of the cathodic type. Thermodynamic

parameters and Flory-Huggins adsorption isotherms described the experimental

findings. The number of active sites, equilibrium constant, enthalpy and change of

free energy were computed for all inhibitors studied. This information suggested that

organic molecules were adsorbed and displaced water molecules from the steel

surface. X-ray photoelectron spectroscopy confirmed that species of N, C and O

interacted with the steel to form a continuous protective film. The effect of cysteine

on the corrosion of low carbon steel in H2SO4 solution was studied using

electrochemical and scanning electron microscopy (SEM) techniques [97].

Electrochemical impedance spectroscopy (EIS) results reveal that the presence of

cysteine at low concentrations (0.1-0.5 mmoL-1) promoted the carbon steel corrosion

process, whereas an inhibiting effect was observed at higher concentrations (1.0-5.0

mmoL-1), which was enhanced on deaeration of the test solution. Polarization results

revealed that cysteine actually inhibited the cathodic process at all concentration but

exerted a stimulating effect on the anodic metal dissolution reaction. Despite the

cathodic inhibiting effect, the polarization resistances at low cysteine concentrations

were less than that in the blank acid. This suggests that anodic reaction was the

predominant influence determining the corrosion rates in the presence of cysteine.

The adsorption characteristics of methionine on mild steel surface in 0.5 M

H2SO4 solution at a temperature of 25 ± 0.1 oC have been investigated using

electrochemical impedance spectroscopy, polarization resistance measurement and

polarization curves measurement techniques [108]. Constant phase element (CPE) has

been used instead of double layer capacitance (Cdl) in circuit that models the double

layer to represent the capacitive semicircle depression in the complex plane plots. The

 

 

29

values of polarization resistance (Rp) from polarization resistance measurement

technique agreed well with the sum of the distinct resistances (solution resistance, Rs

and charge transfer resistance, Rct) from impedance technique. Adsorption of

methionine on mild steel surface was found to obey the Langmuir adsorption isotherm

with a ∆Goads of -32 kJ/mol. The behavior of L-methionine as a green organic

inhibitor was studied using mild steel rotating disc electrode [119]. Electrochemical

impedance spectroscopy (EIS) and polarization measurements were carried out in the

absence and presence of inhibitor in 1 M H2SO4 solution under static conditions and

at different rotation speeds; the inhibitor concentration was 5×10−3 M in all tests. The

open circuit potentials (OCP) versus time were measured. At all studied rotation

speeds; it was found that the OCP shifted toward more positive potentials as the

rotation speed increased. This behavior could be attributed to the enhanced mass

transport of inhibitor molecules from bulk to the metal surface in high rotation rates.

Morad et al [120] investigated the effect of five S-containing amino acids namely N-

acetylcysteine, cysteine, S-benzylcysteine, cystine, methionine on the corrosion of

mild steel in 5% sulfamic acid solution at 40 oC using linear polarization Rp,

potentiodynamic polarization curves and EIS techniques. The compounds are

effective inhibitors and the inhibition efficiency follow the order: N-acetylcysteine

(ACC) > cysteine (RSH) > S-benzylcysteine (BzC) > cystine (RSSR) methionine

(CH3SR). The inhibitors affect the anodic dissolution of steel by blocking the anodic

sites of the surface. EIS measurements indicated that charge transfer is the rate

determining step in the absence and presence of the inhibitors and the steel/solution

interface can be represented by the equivalent circuit Rs (Rct Qdl). Adsorption of RSH,

CH3SR and RSSR follows the Langmuir model while the Temkin isotherm describes

 

 

30

the adsorption of ACC and BzC. From the application of Flory-Huggins isotherm, the

number of water molecules displaced by the adsorbing inhibitor molecules was

estimated. The potential of zero charge pzc of mild steel without and with the

inhibitors is calculated and the mechanism of corrosion inhibition is discussed in the

light of the molecular structure. The inhibition effect of L-cysteine as environmental

friendly corrosion inhibitor for XC18 carbon steel in stirred 2 N sulphuric acid was

investigated by electrochemical techniques at 30-60 oC [99]. The impedance

measurements showed that the inhibition efficiency increased with increase of

inhibitor concentration reaching more than 84% at 500 mg/L; L-cysteine act as mixed

type inhibitor and the adsorption followed the Langmuir isotherm. The values of

adsorption free energy (∆Goads) and activation energies (Ea) reveal a physical

adsorption of the inhibitor on the steel surface. Amino acid L-Leucine was evaluated

as a potential corrosion inhibitor for mild steel in 1 N H2SO4 by galvanostatic

polarization and potentiostatic polarization techniques [121]. The electrochemical

results were supplemented by scanning electron microscopy (SEM) and infrared

studies (IR). The electrochemical polarization results showed that L-Leucine is most

effective at 10-1 M concentration at room temperature. The efficiencies were found to

decrease with decrease in concentration and increase in temperature. Electrochemical

results showed that L-Leucine acts as mixed type of inhibitor (blocks the cathodic and

anodic sites to same extent) which is evident from insignificant shift of open circuit

potential. The potentiostatic polarization data shows that inhibitor is passivating type.

Alternating current (AC) impedance measurements of cystine adsorption at mild

steel/sulphuric acid interface in presence of various concentrations of cystine have

been carried out in the 100 kHz-10 mHz frequency range [122]. The results revealed

 

 

31

that cystine is a good and effective inhibitor for mild steel corrosion in 0.5 M H2SO4.

The percent inhibition efficiency changes with cystine concentration. Changes in

impedance parameters indicated the adsorption of cystine on the mild steel surface,

which was verified by SEM and atomic force microscope (AFM) photographs. The

SEM and AFM photographs showed that the inhibition is due to the formation of the

film on the metal surface through adsorption of cystine molecules. Adsorption of

cystine on mild steel surface was found to obey the Langmuir adsorption isotherm

with a standard free energy of adsorption ∆Goads of -33.2 kJ/mol. Energy gaps for the

interactions between mild steel surface and cystine molecule were found to be close to

each other showing that cystine owns the capacity to behave as both electron donor

and electron acceptor.

The inhibition effects of two amino acids methionine and tyrosine on the

corrosion of iron in 0.1 M HCl solution was studied using electrochemical, FTIR and

quantum chemical techniques [123]. The highest inhibition efficiency of 97.8% was

determined at 100 ppm of methionine. The inhibition efficiency of the compounds

was found to increase with increase in their concentration. Methionine was found to

be more effective than tyrosine. The adsorption of the inhibitors accords with the

Langmuir adsorption isotherm. Quantum chemical calculations performed for

methionine and tyrosine, as well as, their protonated structures, corroborate that both

theoretical and experimental inhibition effect of methionine is better than that of

tyrosine.

The inhibition behavior of low carbon steel in 1 M HCl by L-tryptophan in the

temperature range of 298-328 K was investigated using weight loss experiment and

 

 

32

Tafel polarization curves [102]. All the experimental results showed that L-tryptophan

has excellent corrosion inhibition performance and the most effective concentration of

inhibitor is 1×10-2 mol/L. The Tafel polarization curve results indicate that L-

tryptophan acts more as a cathodic than anodic inhibitor. The adsorption of L-

tryptophan on the surface of low carbon steel obeys the Langmuir adsorption

isotherm. The adsorption behavior of L-tryptophan at Fe surface was also investigated

by the molecule dynamics simulation method and density functional theory. The

results indicated that the L-tryptophan could adsorb firmly on the Fe surface through

the indole ring with п-electrons and nitrogen/oxygen atom with lone-pair electrons in

its molecule. Fu et al [124] investigated the corrosion inhibition behavior of four

selected amino acid namely, L-cysteine, L-histidine, L-tryptophan and L-serine on

mild steel in deaerated 1.0 M HCl electrochemically by Tafel polarization and

electrochemical impedance spectroscopy methods and computationally by the

quantum chemical calculation and molecular dynamics simulation. The order of

inhibition efficiency of these inhibitors follows the sequence: L-tryptophan > L-

histidine > L-cysteine > L-serine. The quantum chemical calculations were performed

to characterize the electronic parameters which are associated with inhibition

efficiency. The molecular dynamics simulations were applied to find the equilibrium

adsorption configurations and calculate the interaction energy between inhibitors and

iron surface. Results obtained from Tafel and impedance methods are in good

agreement. The electrochemical experimental results are supported by the theoretical

data. Amin et al [125] investigated three selected amino acids namely, alanine (Ala),

cysteine (Cys) and S-methylcysteine (S-MCys) as safe corrosion inhibitors for iron in

aerated stagnant 1.0 M HCl using Tafel polarization and impedance measurements.

 

 

33

Results indicate that alanine acts mainly as a cathodic inhibitor, while cysteine and S-

methylcysteine function as mixed-type inhibitors. Cysteine, which contains a

mercapto group in its molecular structure, was the most effective among the inhibitors

tested, while alanine was less effective than S-methylcysteine. The low inhibition

efficiency recorded for S-methylcysteine compared with that of cysteine was

attributed to steric effects caused by the substituent methyl on the mercapto group.

The inhibition and adsorption potentials of four amino acids namely cysteine,

glycine, leucine and alanine for the corrosion of mild steel in 0.1 M HCl solution were

investigated using experimental and quantum chemical approaches [126]. The

experimental study was carried out using gravimetric, gasometric, thermometric and

FTIR methods while the quantum chemical study was carried out using semi-

empirical, ab-initio and quantitative structure activity relation (QSAR) methods. The

results obtained indicated that various concentrations of the studied amino acids

inhibited the corrosion of mild steel in HCl solution through physiosorption. The

inhibition potentials of the inhibitors decreased in the order: cysteine > leucine >

alanine > glycine. The adsorption of the inhibitors on mild steel surface was found to

be exothermic, spontaneous and supported the Langmuir adsorption model. Results

obtained from quantum chemical studies showed excellent correlations between

quantum chemical parameters and experimental inhibition efficiencies. Correlations

between the experimental inhibition efficiencies and theoretical inhibition efficiencies

were also excellent. Fu et al [127] investigated the inhibition effect of L-Cysteine

(CYS) and its derivatives including N-acetyl-L-cysteine (NACYS), N-acetyl-S-

benzyl-L-cysteine (NASBCYS), and N-acetyl-S-hexyl-L-cysteine (NASHCYS) as

green chemical for mild steel in 1 M HCl solution. Weight loss method and Tafel

 

 

34

polarization measurement were performed to determine the corrosion parameters and

inhibition efficiencies. Experimental results showed that these compounds suppressed

both anodic and cathodic reaction, and the inhibition efficiency of the four inhibitors

followed the order: N-acetyl-S-benzyl-L-cysteine > N-acetyl-S-hexyl-L-cysteine > N-

acetyl-L-cysteine > L-cysteine. The relationships between quantum chemical

parameters and corrosion inhibition efficiency were discussed. Molecular dynamics

method was used to simulate the adsorption behavior of each inhibitor in the solvent.

The results showed that these four inhibitors can adsorb on mild steel surface by

donor acceptor interactions between lone-pair electrons of heteroatoms/п-electrons of

aromatic ring and vacant d-orbital of iron. Abdel Ghanyl et al [128] investigated the

inhibition effect of four amino acids glycine, leucine, arginine and valine on the

corrosion of 316L stainless steel in 1.0 M H2SO4 by studying open-circuit potential

and potentiodynamic polarization measurements. Corrosion parameters were

determined by extrapolation of the cathodic and anodic Tafel region. Glycine, leucine

and valine inhibited the corrosion process, whereas the arginine accelerated the

corrosion phenomenon. The presence of arginine at high concentration turns the

surface of 316L stainless steel electrochemically active, probably dissolving the

passivation layer and promoting the stainless steel anodic dissolution. Shenawy [129]

investigated the corrosion inhibition of Lysine on SS 316L in 0.1 M H2SO4 solution

using open circuit potential measurements, potentiodynamic measurements and

scanning electron microscopy (SEM) techniques. The open circuit potential becomes

more positive with increasing the concentration of lysine. Potentiodynamic

polarization measurements showed that the presence of lysine affect the cathodic

process and decrease the corrosion current to a greater extent and corrosion potential

 

 

35

shifts towards more negative values. Results revealed clearly that lysine is a good

cathodic type inhibitor for SS 316L in 0.5 M H2SO4. The effect of tryptophan on the

corrosion behavior of low alloy steel in sulfamic acid which is widely used in various

industrial acid cleaning applications was studied by electrochemical methods

(electrochemical impedance spectroscopy; EIS and the new technique electrochemical

frequency modulation; EFM) as well as gravimetric measurements [130]. It was

shown that the inhibition property of tryptophan was due to the electrostatic

adsorption of the protonated form of tryptophan on the steel surface. Adsorption of

the inhibitor molecule, onto the steel surface followed the Temkin adsorption

isotherm. All of the obtained data from the three techniques were in close agreement,

which confirmed that EFM technique can be used efficiently for monitoring the

corrosion inhibition under the studied conditions. Three selected amino acids, namely

serine, threonine and glutamine were tested as corrosion inhibitors for cold rolled

steel (CRS) in 1.0 M HCl solutions at 283-333 K by Chemical (weight loss) and

electrochemical (Tafel polarization) methods [131]. Electrochemical frequency

modulation (EFM), a non-destructive corrosion measurement technique that can

directly give values of corrosion current without prior knowledge of Tafel constants,

was also employed. Experimental corrosion rates determined by the Tafel

extrapolation method were compared with those obtained by EFM technique and the

weight loss method. Morphologies of the corroded and the inhibited surfaces were

studied by means of atomic force microscopy (AFM) and scanning electron

microscopy (SEM). Tafel plots showed that the three tested amino acids act as mixed-

type inhibitors. The amino acids appeared to function through adsorption following

 

 

36

the Temkin adsorption isotherm model. Glutamine was found to inhibit corrosion of

steel more effectively than serine and threonine.

In addition to iron base alloys amino acids have also been exploited as

corrosion inhibitor for non ferrous metals like Cu, Al, Pb and their alloys [132-143].

Inhibition effect of α-amino acids namely, arginine, histidine, glutamine, aspargine,

alanine and glycine on the inhibition of pitting corrosion of Al in 0.1 M NaCl solution

was studied by potentiodynamic technique [132]. All the studied inhibitors shifted the

pitting potential (Ep) and the protection potential (Epp) towards more noble values.

The order of effectiveness of the inhibitors was arginine > histidine > glutamine >

aspargine > alanine > glycine. Abdallah and Atia [133] reported the corrosion

parameters for the corrosion of admiralty brass electrode in 0.1 M H2SO4 in the

presence of four amino acids glycine, L-phenyl alanine and L-tyrosine and L-histidine

by carrying out weight loss and polarization studies. The inhibition efficiency of these

compounds was found to depend on the concentration and their chemical structure.

The inhibition efficiency was found in the order of histidine > tyrosine > phenyl

alanine > glycine. Ashassi-Sorkhabi et al [95] investigated the inhibition effect of

amino acids alanine, leucine, valine, proline, methionine, and tryptophan towards the

corrosion of aluminum in HCl and H2SO4 by using weight loss, linear polarization and

SEM techniques. The investigated amino acids acted as good inhibitors for the

corrosion of aluminum in HCl and H2SO4 solution. The adsorption of used amino

acids on aluminum surface followed Langmuir and Frumkin isotherms. The inhibition

effect of two amino-acid compounds, DL-alanine and DL-cysteine, on copper

corrosion in an aerated 0.5 mol/L HCl solution was studied by weight-loss

measurements, potentiodynamic polarization curves, and electrochemical impedance

 

 

37

spectroscopy [93]. A conventional benzotriazole (BTA) inhibitor was also tested for

comparison. DL-cysteine was shown to be the most effective inhibitor among the

tested inhibitors. Potentiodynamic polarization results revealed that both the DL-

alanine and DL-cysteine acted as an anodic inhibitor; however, DL-cysteine, in

particular, was more effective, as it strongly suppressed anodic current densities. The

improved inhibition efficiency of DL-cysteine in the 0.5 mol/L HCl solution was due

to its adsorption on the copper surface via the mercapto group in its molecular

structure. Morad and Hermas [134] reported the influence of amino acids glycine,

serine, methionine and vitamin C and some of their binary mixtures on the anodic

dissolution of tin in 3.5% NaCl solution. The inhibitors were used in the concentration

range of 25-100 mM. The studies were carried out by means of potentiodynamic and

impedance techniques. The corroded tin surface was examined by SEM. The results

indicated that the passive behavior of tin is greatly improved by the presence of 50-

100 mM glycine and methionine while such improvement is achieved only at 100 mM

serine. Both cysteine and vitamin C showed aggressive action. The pitting corrosion

of aluminium alloy 7075 in 0.05 M NaCl solution at pH values of 4, 5, 7 and 8 was

studied by potentiostatic method [135]. It was found that addition of 10-2 M hydroxy

carboxylic acids or amino acids to NaCl solutions shifted Epit values to noble

directions. Amino acids were more effective in shifting Epit values to nobler directions

in acidic solutions whereas hydroxy carboxylic acids were more effective in shifting

Epit values to nobler directions in neutral or basic solutions. Moretti and Guidi [136]

investigated the use of tryptophan as a copper corrosion inhibitor in 0.5 M aerated

H2SO4 in the temperature range 20-50 oC. The effectiveness of the inhibitor was

assessed through potentiodynamic (at 1 h, 72 h, 6 months), spectrophotometric (72 h

 

 

38

tests) and gravimetric (72 h tests) tests. At 20-50 oC (1 h tests) the tryptophan

adsorption followed Bockris-Swinkels’ isotherm. The tryptophan even underwent

photodegradation, but this did not affected the inhibition percentage which was 80%

for the solutions kept in the dark as well as those kept in light.

The environmentally safe corrosion inhibitions of lead in aqueous solution

with different pH (2, 7 and 12) were investigated by Helal et al [137]. The corrosion

rate was calculated in the absence and presence of the corrosion inhibitor amino acids

using polarization and impedance technique. Corrosion inhibition efficiency up to

87% was recorded with glutamic acid in neutral solutions. The experimental

impedance data were fitted to theoretical values according to an equivalent circuit

model to explain the behavior of the metal under different conditions. The corrosion

inhibition process was found to depend on the adsorption of the amino acid molecules

on the metal surface. The free energy of adsorption of glutamic acid on Pb was found

to be equal to -2.9 kJ/mol, which reveals that the inhibitor is physically adsorbed on

the metal surface. The inhibition effect of five amino acids namely, valine, glycine,

arginine, lysine and cysteine on the corrosion of copper in nitric solution was studied

by using weight loss and electrochemical polarization measurements [138]. Valine

and glycine accelerate the corrosion process but arginine, lysine and cysteine inhibit

the corrosion phenomenon; cysteine was the best inhibitor attaining IE of 61% at 10-3

M. Ranjana et al [139] investigated the inhibition effect of three amino acids glycine,

L-aspartic acid, L-glutamic acid and their benzenesulphonyl derivatives namely, N-

benzenesulphonyl glycine, N-benzenesulphonyl L-aspartic acid and N-

benzenesulphonyl L-glutamic acid on the corrosion of brass in 0.6 M aqueous sodium

chloride solution studying by potentiodynamic polarization and electrochemical

 

 

39

impedance spectroscopy. It was shown that the inhibition efficiency of these

compounds increases in the order of L-glutamic acid > L-aspartic acid > glycine for

amino acids and same trend is followed for benzenesulphonyl derivatives. The

inhibition efficiency of N-benzenesulphonyl L-glutamic acid with 4×10-5 M

concentration is the highest, which reaches 81.2-85.5%. It has been observed that

introduction of C6H5SO2- group increases the inhibition efficiency of the amino acids

by 14-33% due to п electron contribution of the benzene ring and presence of more

active adsorption sites. Helal and Badawy [140] studied the corrosion inhibition of

Mg-Al-Zn alloy in stagnant naturally aerated chloride free neutral solutions using

amino acids as environmentally safe corrosion inhibitors. The corrosion rate in the

absence and presence of the corrosion inhibitor was calculated using the polarization

technique and electrochemical impedance spectroscopy. It was found that Phenyl

alanine has corrosion inhibition efficiency up to 93% at a concentration of 2×10−3

moldm−3. The free energy of the adsorption process revealed a physical adsorption of

the inhibitor molecules on the alloy surface. Helal [141] showed the corrosion

inhibition behavior of Mg-Al-Zn alloy by different electrochemical techniques in

neutral solutions containing chloride ions at different concentrations. The corrosion

rate was calculated in the absence and presence of methionine as an environmentally

safe corrosion inhibitor using polarization and impedance techniques. The results

reveal that methionine inhibits the corrosion reaction by adsorption onto the metallic

surface. Polarization results revealed that methionine acts as an anodic inhibitor. The

inhibitory effects of tryptophan on the corrosion of Aluminium alloy AA 2024 in 1 M

HCl, 20% (wt. %) CaCl2, and 3.5% (wt. %) NaCl solutions were investigated via

polarization techniques, electrochemical impedance spectroscopy, and weight loss

 

 

40

methods [142]. The scanning electron microscope technique was employed to observe

corrosion morphology. The results suggest that AA 2024 was corroded in these three

corrosive media to some extent and that tryptophan can significantly inhibit the

corrosion of aluminum alloys. The inhibition efficiency increased with increasing

concentrations of tryptophan, and the best inhibition efficiency exhibited was about

87% in 1 M HCl solution with 0.008 M tryptophan. Tryptophan acted as a cathodic

corrosion inhibitor and affected the hydrogen evolution reaction, which was the main

electrode reaction in the 1 M HCl solution. In solutions with 20% CaCl2 and 3.5%

NaCl, tryptophan was adsorbed onto anodic areas, thus increasing the activation

energy of the interface reaction as an anodic corrosion inhibitor. The Dmol3 program

of Material Studio 4.0 was used to obtain the optimized geometry of the tryptophan

inhibitor and some quantum-chemical parameters. Front orbital distributions and

Fukui indices indicate that the molecular active reaction zones were located in the

indole ring of tryptophan. Abdel Rahman et al [143] reported a comparative study of

different types of amino acids namely, proline, cysteine, phenyl alanine, alanine,

histidine and glycine as corrosion inhibitors for copper corrosion in 8 M phosphoric

acid at different temperatures. Potentiodynamic polarization and rotation techniques

were used during the experiments. It is shown that amino acids have strongest

inhibitive effects that provide good protection to copper surface against corrosion in

acid solutions. Adsorption of amino acids on copper surface, in 8 M phosphoric acid

solution, follows Temkin adsorption isotherm model. Quantum chemical parameters

were calculated using semiempirical PM6 method to find a good correlation with the

inhibition efficiency. A good correlation was found between the theoretical

calculations and experimental observations.

 

 

41

1.7 Corrosion Inhibition Using Surfactants

Surfactants also called surface active agents are molecules composed of a

polar hydrophilic group, the ‘‘head”, attached to a non-polar hydrophobic group, the

‘‘tail’’. The hydrophobic portion is usually a branched or straight hydrocarbon chain

containing 6-22 carbon atoms. The hydrophilic portion can be ionic (cationic,

anionic), zwitterionic, or nonionic depending on the nature of their head groups. In

aqueous solution, the hydrophobic chain interacts weakly with the water molecules,

whereas the hydrophilic head interacts strongly via dipole or ion-dipole interactions.

This strong interaction between water and ‘head-group’ renders the surfactant soluble

in water. Surfactants can reduce surface and interfacial tensions by accumulating at

the interface of immiscible fluids and increase the solubility, mobility, bioavailability

and subsequent biodegradation of hydrophobic or insoluble organic compounds.

Depending on the charge of head groups, the surfactants are classified as

anionic, cationic, nonionic and amphoteric (zwitterionic) surfactants. Anionic

surfactants are dissociated in water into an amphiphilic anion, and a cation, which is,

in general, an alkaline metal ion (Na+, K+) or a quaternary ammonium. This class of

surfactants is widely used in industrial applications due to their relatively low cost of

manufacture. Most commonly used anionic groups are alkylbenzenesulfonates, lauryl

sulfate, lignosulfonates, fatty acid soaps, etc. In cationic surfactants, cation is the

surface-active species. The quaternary ammonium salts are the main compounds

belonging to this class of surfactants. A very large proportion of this class of

molecules corresponds to nitrogen compounds such as fatty amine salts and

quaternary ammoniums, with one or several long chain of the alkyl type, often

 

 

42

coming from natural fatty acids. Nonionic surfactant does not have any surface charge

and have either a polyether or a polyhydroxyl unit as the non-polar group. In the

majority of the nonionics, the polar group is a polyether consisting of oxyethylene

units, prepared by the polymerization of ethylene oxide. They can be mixed with

other types of surfactants, e.g., anionic or cationic, to enhance their properties and

reduce surfactant precipitation [144, 145]. This class of surfactants has substantially

lower critical micelle concentrations (CMC) than the corresponding ionic surfactants

and continues to be dominated by ethylene oxide adducts of alkylphenols and fatty

alcohols. The amphoteric or zwitterionic surfactants contain both cationic and anionic

groups and show good compatibility with other surfactants. Some amphoteric

surfactants are insensitive to pH, whereas others are cationic at low pH and anionic at

high pH with an amphoteric behavior at intermediate pH. Some of the examples are

betaines, aminoacids, sulfobetaines and phospholipids. A new class of recently

developed surfactants called Gemini or dimeric surfactants have two hydrophilic

(chiefly ionic) groups and two tails per surfactant molecule. These twin parts of the

surfactants are linked by a rigid or flexible spacer group of varying length (most

commonly a methylene spacer or an oxyethylene spacer). Gemini surfactants exhibit

remarkably a number of superior properties when compared to single chain

conventional surfactants. They possess lower values of CMC, better wetting

properties, increased surface activity and lower surface tension at the CMC, enhanced

solution properties such as hard-water tolerance and lower Krafft points [146-150].

The promising potential application of surfactants as corrosion inhibitors in

acidic media has been widely studied during the last two decades [151-161]. The

corrosion inhibition by surfactant molecules is related to their remarkable ability to

 

 

43

influence the properties of surfaces and interfaces. In general, in aqueous solution the

inhibitory action of surfactant molecules may be due to the physiosorption

(electrostatic) or chemisorption onto the metallic surface, depending on the charge of

the solid surface. The degree of their adsorption on metal surfaces depend on the

nature of the metal, the mode of adsorption, chemical structure of the surfactant, the

metal surface condition, and the type of corrosion media [162]. The adsorbed

surfactants molecules form a monolayer or bilayer, hemimicelles or admicelles and

prevent the acids to attack the surfaces. Surfactant inhibitors have many advantages

over traditional corrosion inhibitors. For example, they are easily produced, are

economical and possess high inhibition efficiency and low toxicity [163-165]. The

adsorption of a surfactant markedly changes the corrosion resisting property of a

metal, and for these reasons, studies of the relation between adsorption and corrosion

inhibition are of considerable importance [166-169] The adsorption behavior of

surfactants at the solid/solution interface is somewhat similar to that at the

gas/solution interface although the latter is more complicated than the former [170,

171]. Surfactants have widely been used for inhibition of pitting corrosion of stainless

steel [172], as corrosion inhibitors for carbon steel pipelines in oil fields, aluminum

alloys and mild steel [158, 173, 174]. A number of investigators were able to reduce

the steel corrosion in acidic media [175-180] by means of the surfactants. The

surfactant can be used either alone or in mixture with other compounds to improve

their performance as inhibitors.

The ability of a surfactant molecule to adsorb is generally directly related to its

ability to aggregate and to form micelles. Micelles are spontaneously formed clusters

of surfactant molecules (typically 40 to 200), whose size and shape are governed by

 

 

44

geometric and energetic considerations. Consequently, the CMC is an important

parameter in determining the effectiveness of a surfactant as a corrosion inhibitor. The

concentration above which the surfactant molecules aggregate to form micelles is

called critical micelle concentration or simply CMC. When the concentration of

surfactant adsorbed on the solid surface is high enough, organized structures (hemi-

micelles such as bi- or multilayer) are formed, which decrease the corrosion reaction

by blocking the metallic surface [156, 179, 181]. Below the CMC, individual

surfactant molecules or monomers tend to adsorb on exposed interfaces, so interfacial

aggregation reduces surface tension; this is related to corrosion inhibition. Above the

CMC, the surface becomes covered with more than one monolayer and forms a

protection layer on the metal surface. Thus any additional surfactant added to the

solution above the CMC will lead to the formation of micelles or multiple adsorbed

layers on the surface. Consequently, the surface tensions, and also the corrosion

current density, are not altered significantly above the CMC. Therefore, an efficient

surfactant inhibitor is one that aggregates or adsorbs at low concentrations. In general,

lower the CMC of the surfactant the greater is its tendency to adsorb at the solid

surface [182-189]. Various factors effects the CMC values e.g., temperature, the

length of the hydrocarbon tail, the nature of the counter ions and the existence of salts

and organic additives, and thus amphiphiles have characteristic CMC values under

given conditions [190, 191].

1.8 Synergism Consideration in Corrosion Inhibition

The word synergism is derived from the Greek word synergos meaning acting

together. Synergism or mutual increase is a combined action of compounds greater in

 

 

45

total effect than sum of individual effects [192-196]. It is one of the most important

effects in inhibition process and serves as basis for modern corrosion inhibiting

formulations [197, 198]. Synergism of corrosion inhibitors is either due to interaction

between constituents of the inhibitor or due to interaction between the inhibitor and

one of the ions present in aqueous solution [199-201]. It is a nonlinear effect resulting

in non-additive efficiencies of inhibitor components. In most cases, the mechanism of

synergism differs from the mechanism of the individual inhibitors. It is, therefore,

necessary for corrosion researchers to discover, explore and use synergism in the

complicated corrosive media. By taking advantage of synergism the amount of

inhibitor applied can be decreased or an environmental friendly but less effective

corrosion inhibitor can be used more effectively.

The addition of halide ions to organic compounds has shown synergistic effect

and resulted in improved inhibition efficiency of many organic compounds [197, 202-

205]. However, there remains relatively few works directed towards the synergistic

between the different amino acids and other compounds. Oguzie et al [98] found that

inhibition efficiency of methionine synergistically increased in the presence of KI,

with an optimum KI/methionine ratio of 1/1. Ismail [206] reported that the maximum

inhibition efficiency of cysteine can be achieved at about 84%; the presence of Cu2+

ions synergistically increases the inhibition efficiency to 90% in neutral and acidic

chloride solution. The influence of Zn2+ ions on inhibition efficiency of methionine

for copper corrosion in 0.5 M HCl has been studied by cyclic voltammetry,

electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization

techniques [207]. Methionine has shown limited inhibiting properties for copper

corrosion in 0.5 M HCl. However, the presence of Zn2+ ions synergistically increases

 

 

46

the inhibition efficiency to 92%. Results obtained from potentiodynamic polarization

and impedance measurements are in good agreement. The corrosion protection of

copper by glutamic acid, cysteine, glycine and their derivative (glutathione) in 0.5 M

HCl solution has been studied by electrochemical impedance spectroscopy and cyclic

voltammetry [208]. The inhibition efficiency of the organic inhibitors on copper

corrosion increase in the order: glutathione > cysteine > cysteine + glutamic acid +

glycine > glutamic acid > glycine. Maximum inhibition efficiency for cysteine reaches

about 92.9% at 15 mM concentration level. The glutathione can give 96.4% inhibition

efficiency at a concentration of 10 mM. The intramolecular synergistic effect of

glutamic acid, cysteine and glycine moieties in glutathione is attributed to the lower

energy of the lowest unoccupied molecular orbital (ELUMO) level and to the excess

hetero - atom adsorption centers and the bigger coverage on the copper surface.

Though the synergistic influence of halide ions on different organic

compounds has been adequately investigated there remains relatively few works

directed towards the synergistic between the different organic compounds and

surfactants [209, 210]. In a very recent paper the inhibition behavior of methionine

combined with cetrimonium bromide (CTAB) and cetylpyridinium bromide (CPB)

for Cu corrosion in 0.5 M HCl solution has been reported. It has been shown that

combination of methionine with CTAB or CPB provides strong synergistic inhibition

effect [211].

 

 

47

1.9 STATEMENT OF THE WORK PRESENTED IN THE THESIS

The corrosion of mild steel is a subject of fundamental, academic and

industrial concern and has received considerable attention during the last few decades.

The use of inhibitors is one of the most effective practical and economic methods to

protect metallic surfaces against corrosion in aggressive acidic media. Mineral acids

are widely used in various industries for pickling of steel at elevated temperatures up

to at 60 oC. This technique besides being used to remove corrosion scales from steel

surface without causing acid attack of the bulk metal is also effectively applied in

cleaning of industrial equipment and acidization of oil well. Most of the efficient

pickling inhibitors are organic compounds containing hetero atoms such as sulfur,

nitrogen, oxygen, phosphorus, and multiple bonds or aromatic rings in their

structures. However, most of the organic compounds used as corrosion inhibitors are

toxic and hazardous to both human beings and the environment and needs to be

replaced by nontoxic, environment friendly compounds. As a result, the current

research trend is towards the development of nontoxic, economical and more

environmentally safe green chemicals as corrosion inhibitors.

It has been shown by a number of investigators that some amino acids can act

as corrosion inhibitors, which has generated an increasing interest in these

compounds. Amino acids are attractive as corrosion inhibitors because they are

relatively easy to produce with high purity at low cost and are nontoxic,

biodegradable and completely soluble in aqueous media. Surfactants, which have a

remarkable ability of influencing the surfaces and interfaces, have shown a significant

role in the inhibition of acid corrosion of mild steel. The surfactant can be used either

alone or in mixture with other compounds to improve their performance as inhibitors.

 

 

48

A survey of literature indicates that the corrosion inhibition effect of amino acids in

presence of surfactants perhaps has yet not been reported. Amino acids are likely to

interact with the surfactants to form complex structure and help to adhere to mild

surface and thus offer greater resistance to corrosion.

The work presented in this thesis deals with the corrosion inhibition behavior

of different amino acids separately and in combination with very low concentration of

the anionic and cationic surfactants on mild steel in 0.1 M H2SO4/ HCl solutions. The

aim of the surfactants addition was to improve the corrosion inhibition behavior of

environment friendly but less effective amino acids as corrosion inhibitor for mild

steel corrosion in acidic medium. The concentration of surfactants was intentionally

kept low so that the green nature of amino acids is least influenced. The amino acids

considered for the investigations are L-Histidine (LHS), L-Tryptophan (LTP), L-

Methionine (LMT), and L-Cystine (LCY). The surfactants subjected to investigation

are anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant

cetyltrimethyl ammonium bromide (CTAB). The techniques used are weight loss

measurements, potentiodynamic polarization measurements, scanning electron

microscopy (SEM), atomic force microscopy (AFM) and thermodynamic/kinetic

parameters. The breakup of the work contained in various chapters is as follows:

Chapter I

This chapter deals with the general introduction. The general introduction

briefly describes the fundamentals of corrosion which includes the definition of

corrosion and its importance, cost of corrosion and method of corrosion control. It

presents an exhaustive literature survey exploiting green compounds including amino

acids as corrosion inhibitor for mild steel in acid solutions. Except for some early

 

 

49

pioneering research papers, the thesis include literature survey from the selected

research papers, reviews and reports published on the subject during the last three

decades. Special emphasis has been laid to the work which has direct or indirect

bearing on the studies presented in this thesis. It might be possible that some results of

the important studies have been left unquoted quite inadvertently, yet there was

absolutely no intension to undermine those works.

Chapter II

This chapter deals with the experimental details which includes the materials

and methods used during the experimental work. The details of corrosion tests which

have been undertaken to investigate the inhibition behavior of amino acids have also

been explained in this chapter.

Chapter III

The work presented in this chapter deals with the corrosion inhibition behavior

of L-histidine alone and in presence of surfactants, sodium dodecyl sulfate (SDS) and

cetyltrimethyl ammonium bromide (CTAB) on mild steel in 0.1 M H2SO4 solution at

30-60 °C. The corrosion inhibition efficiency of L-histidine in acid solution alone and

in presence of SDS and CTAB was determined using weight loss and

potentiodynamic polarization measurements. The surface morphology of the mild

steel before and after immersion in 0.1 M H2SO4 solution was also examined using

SEM and AFM studies.

Chapter IV

This chapter deals with the corrosion inhibition behavior of L-tryptophan and

surfactants additives on mild steel in 0.1 M HCl in the temperature range of 30-50 °C.

The corrosion performance of additives on mild steel corrosion was investigated by

 

 

50

weight loss method and potentiodynamic polarization techniques. To determine if the

corrosion inhibition is due to the formation of a protective film by adsorption of

inhibitors scanning electron spectroscopy has also been performed on the corroded

mild steel specimens. The mode of adsorption of inhibitor molecules on mild steel

surface was also elucidated by determination of thermodynamic/kinetic parameters.

Chapter V

This chapter deals with the corrosion inhibition behavior of sulfur containing

amino acid L-methionine with surfactants mixture. The corrosion behavior of mild

steel in 0.1 M H2SO4 in absence and presence of different concentrations of L-

methionine alone and in combination with surfactants SDS and CTAB, was studied in

the temperature range of 30-60 oC using weight loss and potentiodynamic polarization

measurements. The corrosion protection of mild steel by L-methionine and surfactants

additives was confirmed by scanning electron spectroscopy and atomic force

microscopy techniques and determination of thermodynamic/kinetic parameters.

Chapter VI

This chapter presents the results of the investigation concerning with the

corrosion inhibition behavior of L-cystine and surfactant additives on mild steel

corrosion in 0.1 M H2SO4 in the temperature range of 30-60 °C. The corrosion

performance was investigated by weight loss measurements, potentiodynamic

polarization measurements, scanning electron spectroscopy and atomic force

microscopy techniques.

 

 

51

FIGURE CAPTIONS

Figure 1.1 Phase diagram of iron

Figure 1.2 Cathodic protection by impressed current

Figure 1.3 Cathodic protection by sacrificial anode

Figure 1.4 Schematic diagram showing protection range and optimum potential

for anodically protecting an active-passive metal

 

 

 

 

 

 

 

 

 

 

 

52

 

 

    Figure 1.1

 

 

E/V

Immunity Fe

Corrosion Fe2+

Fe3+

FeO42-

Fe2O3.nH2O

 

 

53

 

 

 

 

 

 

 

 

Figure 1.2 

 

 

54

 

 

 

Figure 1.3 

3O2 + 6H2O

12OH–

4Al+++

12e– 

 

 

55

 

 

 

 

 

Figure 1.4